Stick-type ignition coil having improved structure against crack or dielectric discharge

ABSTRACT

A stick-type ignition coil have a central core, a cylindrical member, primary spool, primary coil, secondary spool, secondary coil, outer core and a resin insulator. The two longitudinal end corners and faces of the core are covered by respective buffer members. The inner circumferential corners of the outer core is supported by ring members. Some of the members disposed radially inside and other members disposed radially outside of the inside members are held slidably to each other in the ignition coil. The spools are made of resin containing a rubber in excess of 5 weight percent and reinforcing materials. The resin insulator contains a flexible material.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of Ser. No. 09/023,613 filed Feb. 13,1998 now U.S. Pat. No. 6,208,231.

This application relates to and incorporates herein by referenceJapanese Patent Application Nos. 9-30403, 9-30404, 9-110836, 9-173947,9-213626, 9-214939, 9-214940, 9-214941, 9-214943, 9-357011 and 9-357143,filed on Feb. 14, 1997, Feb. 14, 1997, Apr. 28, 1997, Jun. 30, 1997,Aug. 7, 1997, Aug. 8, 1997, Aug. 8, 1997, Aug. 8, 1997, Aug. 8, 1997,Dec. 25, 1997 and Dec. 25, 1997, respectively.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an ignition coil for an internalcombustion engine and, more particularly, to a stick-type ignition coilto be fitted directly in the plug hole of an internal combustion engine.

2. Description of Related Art

As an ignition coil, a stick-type ignition coil is known. It has arod-shaped central core disposed in a housing, and a primary coil and asecondary coil wound respectively on a primary spool and a secondaryspool made of resin. Resin is filled in the housing of the ignition coilas an electric insulator. The insulator not only provides electricinsulation among individual members in the housing but also fillsclearances between wires of the coils thereby to restrict movement orbreakage of the coils which may arise from engine vibrations. As theinsulator, a thermosetting resin such as epoxy is used in considerationof the heat resistance. The ignition coil further has a permanent magnetattached to at least one of the two longitudinal ends of the centralcore to raise a voltage to be supplied to a spark ignition plug.

In this type of ignition coil, the central core contacts not only theresin insulator but also a case member such as a spool enclosing theouter circumference of the central core. The central core and the resininsulator or the case member, as having different thermal expansioncoefficients, may repeatedly expand and contract as the surroundingtemperature rises and falls. Then, the resin insulator or the casemember, as contacting with the central core, especially the resininsulator or the case member contacting the longitudinal end corners ofthe central core, may crack, which results in defective electricinsulation.

When the resin insulator or the case member around the central corecracks, an electric discharge may occur through the cracks between thesecondary coil or a high voltage terminal (high voltage side) and thecentral core (low voltage side). If the discharge occurs between thehigh voltage side and the central core, the electric insulation betweenthe high voltage side and the central core is broken to lower thevoltage to be generated in the secondary coil, thus disabling ageneration of desired high voltage.

If the central core and the resin insulator or the case memberrepeatedly expand and the contract due to changes in the temperature,the central core is caused to receive a load in the radial direction andin the longitudinal direction from the resin insulator and the casemember due to the difference in the thermal expansion coefficient.Especially when the central core receives the load in the longitudinaldirection, the magnetic permeability of the core may drop causingmagneto-striction which disables generation of a required high voltage.

It is desired in a stick-type ignition coil to dispose an outer corearound the outer periphery of the primary spool and the secondary spool.Since this outer core contacts directly with the insulator in thehousing, the outer core and the insulator having different thermalexpansion coefficients, may repeat expansions and contractions as thetemperature changes. As a result, the insulator contacting with theouter core may crack causing an electric discharge between the secondarycoil or a high voltage terminal the outer core. This discharge lowersthe high voltage to be applied to the ignition plug.

In another ignition coil disclosed in Japanese Utility Model PublicationNo. 59-30501, although not a stick-type, the corners of the core arecovered by over-coating the surface of the core with an elastomer. Thisprevents the corners of the core and the insulator made of epoxy resinfrom coming into direct contact with each other and suppresses thecracks in epoxy resin in the vicinity of the corners of the core. Thisover coating is not applicable to the stick-type ignition howeverbecause the stick-type is so regulated in its external diameter as tomatch the internal diameter of the plug hole.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an ignition coilcapable of suppressing drawbacks caused by changes in surroundingtemperature.

It is another object of the invention to provide an ignition coilcapable of suppressing cracks from occurring in the vicinity of thelongitudinal end corners of the a central core and/or outer core.

It is a further object of the invention to provide an ignition coilcapable of suppressing dielectric breakdown caused by changes insurrounding temperature.

According to the first aspect of the invention, an ignition coil has anelastic buffer member at at least one of longitudinal end corners of acentral core to absorb a difference in thermal expansion coefficientsbetween the central core and a resin insulator or a case member such asa spool. As a result, even if the resin insulator or the case memberhaving the thermal expansion coefficient different from that of thecentral core repeatedly expands and contracts together with the centralcore as the temperature changes, the resin insulator and the case memberin the vicinity of the longitudinal end corners of the central core canbe prevented from cracking. Alternatively, at least one of the two endcorners of the central core may be surrounded by a space, so that a casemember such as a spool or a resin insulator enclosing the outercircumference of the central core is not in contact with thelongitudinal end corners of the central core.

According to the second aspect of the invention, an ignition coil has anangled member to cover the inner circumference corner of thelongitudinal end of an outer core which is arranged around the outercircumferences of a primary coil and a secondary coil, so that a resininsulator is restricted from coming into direct contact with the innercircumference corner of the outer core. As a result, even if the outercore and the resin insulator, having different expansion coefficients,repeatedly expands and contracts as the temperature changes, cracks canbe suppressed in the resin insulator in the vicinity of the innercircumference corner of the outer core. As a result, the electricdischarge can be suppressed so that the drop in the voltage to beapplied to an ignition plug can be restricted. Alternatively, the spoolmay have a flange to be arranged to cover the longitudinal end corner ofthe outer core, so that the cracks, if caused in the resin insulator inthe vicinity of the inner circumference corner of the outer core, willhardly extend to the inner circumference because of being shielded bythe outer spool. As a result, the cracks are less likely to reachelectric wires connecting the coils and terminals in the ignition coilelectrically.

According to the third aspect of the invention, an ignition coil has aseparating member to separate a spool and a resin insulator from eachother so that the spool and the resin insulator disposed inside andoutside of the separating member can expand/contract separately fromeach other with a change in temperature. Thus, the spool and the resininsulator are prevented from cracking in a peripheral part on which alarge force is liable to act.

According to the fourth aspect of the invention, a resin material usedfor at least an inner one of a primary spool and a secondary spoolcontains more than 5 weight % of rubber component. Accordingly, even ifthe inner spool is hindered from contracting toward the inside more thana coil wound thereon in low temperature by adhesion, it can reduce thedistortion and can extend while maintaining the adhesion with the coil,thereby restricting the inner spool from cracking.

According to the fifth aspect of the invention, an ignition coil has aninsulator made of a flexible material to hold individual members adheredto one another even if the members having different thermal expansioncoefficients expand and contract as the temperature changes. Preferably,an average of the thermal expansion coefficient at −40° C. to 130° C. isset within a range of 10 to 30 ppm in a test method corresponding toASTMD790, so that a thermal expansion coefficient of the insulatorbecomes close to that of iron or copper used for a core or coils, thusrestricting distortion of spools and the insulator.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome more apparent from the following detailed description withreference to the embodiments shown in the accompanying drawings. In thedrawings:

FIG. 1 is a longitudinal sectional view showing an ignition coilaccording to the first embodiment of the invention;

FIG. 2 is a sectional view showing a cylindrical member used in thefirst embodiment;

FIG. 3 is an enlarged sectional view showing one end portion of theignition coil according to the first embodiment, the one portion beingdesignated by a circle III in FIG. 1;

FIG. 4 is an enlarged sectional view showing the other end portion ofthe ignition coil according to the first embodiment, the other portionbeing designated by a circle IV in FIG. 1;

FIG. 5 is a longitudinal sectional view showing an ignition coilaccording to the second embodiment of the invention;

FIG. 6 is an enlarged sectional view showing one end portion of theignition coil according to the third embodiment;

FIG. 7 is an enlarged sectional view showing the other end portion ofthe ignition coil according to the third embodiment;

FIG. 8 is an enlarged sectional view showing one end portion of anignition coil according to the fourth embodiment;

FIG. 9 is an enlarged sectional view showing the other end portion ofthe ignition coil according to the fourth embodiment;

FIG. 10 is a sectional view showing an ignition coil according to thefifth embodiment of the invention;

FIG. 11 is an enlarged sectional view showing a low voltage side of theignition coil according to the fifth embodiment;

FIG. 12 is a sectional view showing a high voltage side of the ignitioncoil according to the fifth embodiment;

FIG. 13 is an enlarged sectional view showing the low voltage side of anignition coil according to a sixth embodiment of the invention;

FIG. 14 is an enlarged sectional view showing the low voltage side of anignition coil according to a seventh embodiment of the invention;

FIG. 15 is an enlarged sectional view showing the low voltage side of anignition coil according to a modification of the seventh embodiment;

FIG. 16 is a transverse sectional view showing an ignition coilaccording to the eighth embodiment of the invention;

FIG. 17 is an enlarged sectional view of a part of the ignition coilaccording to the eighth embodiment, the view being taken along a lineXVII—XVII in FIG. 16;

FIG. 18 is a front view showing a primary spool used in the eighthembodiment;

FIG. 19 is a perspective view showing a film on the primary spool usedaccording to a variation of the eighth embodiment;

FIG. 20 is a perspective view showing the film on the primary spoolaccording to another variation of the eighth embodiment;

FIG. 21 is a transverse sectional view showing an ignition coilaccording to the ninth embodiment of the invention;

FIG. 22 is an enlarged sectional view showing a part of the ignitioncoil according to the ninth embodiment, the view being taken alongXXII—XXII in FIG. 21;

FIG. 23 is a longitudinal sectional view showing an ignition coilaccording to the tenth embodiment of the invention;

FIG. 24 is a transverse sectional view showing a coil wire of a primarycoil before winding according to the tenth embodiment;

FIG. 25 is a longitudinal sectional view showing an ignition coilaccording to the eleventh embodiment of the invention;

FIG. 26 is an enlarged sectional view showing a part of the eleventhembodiment shown in FIG. 25;

FIG. 27 is a perspective view showing a mold die for molding the spoolin the eleventh embodiment;

FIG. 28 is a diagrammatic top view showing a flow of resin within themold die shown in FIG. 27;

FIG. 29 is a characteristic chart showing an effect of the eleventhembodiment;

FIG. 30 is a transverse sectional view showing an ignition coilaccording to the twelfth embodiment of the invention;

FIG. 31 is a sectional view showing a part of the twelfth embodimentshown in FIG. 30;

FIG. 32 is a transverse sectional view showing an ignition coilaccording to the thirteenth embodiment of the invention;

FIG. 33 is a sectional view showing a part of the thirteenth embodimentshown in FIG. 32;

FIG. 34 is a characteristic chart showing an effect of the thirteenthembodiment;

FIG. 35 is a longitudinal sectional view showing an ignition coilaccording to the fourteenth embodiment of the invention;

FIG. 36 is a graph showing a cold distortion of the secondary spoolagainst the characteristic change of the insulator in the fourteenthembodiment;

FIG. 37 is a graph showing a relation between the temperature andexpansion of the insulator in the fourteenth embodiment; and

FIG. 38 is a longitudinal sectional view showing an ignition coilaccording to the fifteenth embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will be described with reference to variousembodiments throughout which the same or like parts are designated bythe same or similar reference numerals.

First Embodiment

An ignition coil 10 is fitted, as shown in FIG. 1, in a plug hole (notshown) which is formed in each cylinder head of an internal combustionengine, and is electrically connectable to a spark ignition plug.

The ignition coil 10 has a cylindrical housing 11 made of a resin, inwhich an accommodating chamber 11 a is formed to accommodate a centralcore assembly 13, a secondary spool 20, a secondary coil 21, a primaryspool 23, a primary coil 24 and an outer core 25. The central coreassembly 13 is comprised of a core 12, and permanent magnets 14 and 15arranged at the two longitudinal ends (top and bottom) of the core 12.An epoxy resin 26 filled in the accommodating chamber 11 a infiltratesbetween the individual members of the ignition coil 10 to ensure theelectric insulations among the members as a resin insulating material.

The core 12 having a column shape is provided by laminating a thinsilicon (Si) steel sheet radially to have a generally circulartransverse section. The permanent magnets 14 and 15 are magnetized tohave a magnetic polarity in the direction opposed to the direction ofthe magnetic flux which is generated by magnetizing the coils. On theother hand, the outer circumference of the core 12 is covered with acylindrical member 17 made of rubber acting as a first buffer member. Onthe permanent magnet 14 covered with the cylindrical member 17,moreover, there is fitted a cap 19 having a through hole. The cap 19 andthe secondary spool 20 construct a case member enclosing the outercircumference of the central core assembly 13.

The cylindrical member 17 is integrally formed into a cylindrical tubeshape, as shown in FIG. 2. The cylindrical member 17 is comprised of acylindrical part 17 a, annular or ring parts 17 b and 17 c formed at thetwo longitudinal ends (top and bottom) of the cylindrical part 17 a andhaving through holes 18 formed at their centers, and angled parts 17 dformed at corners between the cylindrical part 17 a and the annularparts 17 b and 17 c. As shown in FIGS. 3 and 4, the cylindrical part 17a covers the outer circumference of the central core assembly 13, theannular parts 17 b and 17 c cover the portions of the two longitudinalend faces of the central core assembly 13, and the angled parts 17 dcover the end corners of the permanent magnets 14 and 15 or the two endcorners of the central core assembly 13. The annular parts 17 b and 17 care made thicker than the cylindrical part 17 a to function as a secondbuffer member. The through holes 18 are made diametrically smaller thanthe permanent magnets 14 and 15 so that the core 12 and the permanentmagnets 14 and 15 are fitted into the cylindrical member 17 by expandingdiametrically the through holes 18.

As shown in FIGS. 1, 3, and 4, the secondary spool 20 is arranged on theouter circumference of the cylindrical member 17 and is molded of aresin material into such a bottomed cylinder as is closed at thelongitudinal end side of the permanent magnet 15. The secondary coil 21is wound on the outer circumference of the secondary spool 20, and adummy coil 22 is further wound by one turn on the higher voltage side ofthe secondary coil 21. The dummy coil 22 connects the secondary coil 21and a terminal plate 40 electrically. Since the secondary coil 21 andthe terminal plate 40 are electrically connected through not a singlebut the dummy coil 22, the surface area of the electrically connectedportion between the secondary coil 21 and the terminal plate 40 isenlarged to avoid the concentration of electric field at theelectrically connected portion.

The primary spool 23 is arranged on the outer circumference of thesecondary coil 21 and is molded of a resin material. The primary coil 24is wound on the outer circumference of the primary spool 23. A switchingcircuit (not shown) for supplying a control signal to the primary coil24 is disposed outside of the ignition coil 10, and the primary coil 24is electrically connected with the switching circuit through a terminalwhich is insert-molded on a connector 30.

The outer core 25 is mounted on the outer circumference side of theprimary coil 24. The outer core 26 is provided by winding a thin silicon(Si) steel sheet into a cylindrical shape but does not connect thestarting end and the terminal end of the winding to leave a gap in thelongitudinal direction. The outer core 25 has a longitudinal length fromthe outer circumference position of the permanent magnet 14 to the outercircumference position of the permanent magnet 15 to form a magneticcircuit.

A high voltage terminal 41 is insert-molded below the housing 11. Thecentral portion of the terminal plate 40 is folded in the direction toinsert the high voltage terminal 41 to form a pawl. The high voltageterminal 41 is electrically connected with the terminal plate 40 byinserting the leading end of the high voltage terminal 41 into the pawl.The wire of the dummy coil 22 at the high voltage end is electricallyconnected with the terminal plate 40 by fusing or soldering. A conductorspring 42 is electrically connected with the high voltage terminal 41and with the ignition plug when the ignition coil 10 is inserted intothe plug hole. In the open end of the housing 11 at the high voltageside, there is mounted a plug cap 43 made of rubber, into which theignition plug is inserted. When the control signal is fed from theswitching circuit to the primary coil 24, a high voltage is generatedand is applied to the ignition plug through the dummy coil 22, theterminal plate 40, the high voltage terminal 41 and the spring 42.

In the ignition coil 10, the secondary spool 20 and the epoxy resin 26,as enclosing the central core assembly 13, have a thermal expansioncoefficient different from that of the core 12 and the permanent magnets14 and 15, as constructing the central core assembly 13. Usually, thethermal expansion coefficient of the secondary spool 20 and the epoxyresin 26 is larger than that of the central core assembly 13. As aresult, if the central core assembly 13 is not covered with thecylindrical member 17 and if the secondary spool 20 and the epoxy resin26 are in direct contact with the central core assembly 13, thesecondary spool 20 contacting with the central core assembly 13 and theepoxy resin 26 may be cracked by the repeated expansions andcontractions of the central core assembly 13, the secondary spool 20 andthe epoxy resin 26 according to the temperature change. Especially thesecondary spool 20 in contact with the end corners of the permanentmagnets 14 and 15 and the epoxy resin 26 are liable to crack. When thesecondary spool 20 in contact with the end corners of the permanentmagnets 14 and 15 and the epoxy resin 26 crack, an electric dischargemay occur through the cracks between the dummy coil 22, the terminalplate 40 or the high voltage terminal 41 at the high voltage side of thesecondary coil 21 or the high voltage side and the central core assembly13 or the low voltage side. If this discharge occurs between the highvoltage side and the central core assembly 13, the insulation betweenthe high voltage side and the central core assembly 13 is broken tolower the voltage to be generated at the secondary coil so that thedesired high voltage cannot be applied to the ignition plug.

In the first embodiment, however, the outer circumference of the centralcore assembly 13 and the end corners of the permanent magnets 14 and 15are covered with the cylindrical member 17 which is an elastic member sothat the outer circumference of the central core assembly 13 and the endcorners of the permanent magnets 14 and 15 are prevented from cominginto direct contact with the secondary spool 20 and the epoxy resin 26.Even if the central core assembly 13 and the secondary spool 20 or theepoxy resin 26 having different thermal expansion coefficients repeatexpansions and contractions in accordance with the temperature change,moreover, the cylindrical member 17 can elastically deform to absorb thedifference in the thermal expansion coefficients. As a result, thecracks are prevented around the outer circumference of the central coreassembly 13 and especially at the secondary spool 20 and the epoxy resin26 in the vicinity of the two end corners of the central core assembly13, where the cracks might otherwise be liable to occur, so that theelectric discharge between the high voltage side and the central coreassembly 13 can be prevented. This makes it possible to apply thedesired high voltage to the ignition plug.

The thermal expansion coefficient of the cap 19, the secondary spool 20and the epoxy resin 26 is different from or larger than that of thecentral core assembly 13 comprised of the core 12 and the permanentmagnets 14 and 15. As the temperature lowers, therefore, the cap 19, thesecondary spool 20 and the epoxy resin 26 contact to activate a force tocontract the central core assembly 13 in the radial direction and in thelongitudinal direction. Especially when the force is applied in thelongitudinal direction of the central core assembly 13, amagneto-striction to lower the magnetic permeability of the core 12 mayoccur to lower the voltage to be generated in the secondary coil 21.Since the central core assembly 13 is covered at its outer circumferencewith the cylindrical part 17 a and partially at its two longitudinalends with the annular parts 17 b and 17 c thicker than the cylindricalmember 17, however, this cylindrical member 17 is elastically deformedto buffer the forces to be received by the central core assembly 13 inthe radial direction and in the longitudinal direction so that nomagneto-striction occurs in the core 12. As a result, the desired highvoltage can be applied to the ignition plug.

The permanent magnets 14 and 15 are arranged in the first embodiment atthe two longitudinal ends of the core 12, but the permanent magnet maybe arranged at only one end of the core 12.

Second Embodiment

In the second embodiment shown in FIG. 5, no the permanent magnets arearranged at the two longitudinal ends of the core 12, but the core 12itself provides the central core assembly 13. The core 12 is coveredpartially at the outer circumference, at the two end corners and at thetwo longitudinal end faces with the cylindrical member 17.

In the second embodiment, too, the cracks can be prevented around theouter circumference of the core 12 and especially at the secondary spool20 and the epoxy resin 26 in the vicinity of the two end corners of thecore 12, where the cracks might otherwise be liable to occur, so thatthe electric discharge between the high voltage side and the centralcore assembly 13 can be prevented. As a result, the desired high voltagecan be applied to the ignition plug.

As a result of the elastic deformation of the cylindrical member 17,moreover, the forces for the core 12 to receive in the radial directionand in the longitudinal direction are buffered to establish nomagneto-striction in the core 12. Thus, the desired high voltage can beapplied to the ignition plug.

Third Embodiment

In the third embodiment shown in FIGS. 6 and 7, the cylindrical member17 made of rubber to act as the first buffer member is comprised of thecylindrical part 17 a, an angled part 17 b and a bottom disc part 17 cacting as a second buffer member, and is shaped into a bottomedcylindrical shape, as closed at the bottom longitudinal end side of thepermanent magnet 15. The cylindrical part 17 a covers the outercircumference of the central core assembly 13, the annular angled part17 b covers the end corner of the permanent magnet 15, and the disc part17 c covers the bottom end face of the permanent magnet 15. Thecylindrical member 17 is extended upwardly at the side of the permanentmagnet 14 over the end face of the permanent magnet 14. A plate member17 e made of rubber to act as the first buffer member and the secondbuffer member is formed into a disc shape separate from the cylindricalmember 17 and has a larger diameter than the permanent magnet 14. Theend corner of the permanent magnet 14 is covered with the cylindricalmember 17 and the plate member 17 e, and the longitudinal top end faceof the permanent magnet 14 is covered with the plate member 17 e.Moreover, this plate member 17 e effects a sealing between the cap 19acting as the case member and the permanent magnet 14 so that the epoxyresin 26 will not enter the central core assembly 13.

In the third embodiment, too, the cracks can be prevented around theouter circumference of the central core assembly 13 and especially atthe secondary spool 20 and the epoxy resin 26 in the vicinity of the twoend corners of the central core assembly 13, where the cracks mightotherwise be liable to occur, so that the electric discharge between thehigh voltage side and the central core assembly 13 can be prevented. Asa result, the desired high voltage can be applied to the ignition plug.

As a result of the elastic deformations of the cylindrical member 17 andthe plate member 17 e, moreover, the forces for the central coreassembly 13 to receive in the radial direction and in the longitudinaldirection are buffered to establish no magneto-striction in the centralcore assembly 13. As a result, the desired high voltage can be appliedto the ignition plug.

The first buffer member is comprised of the cylindrical member 17 andthe plate member 17 e, and the cylindrical member 17 is formed into thebottomed cylindrical shape having no longitudinal end face at itslongitudinal top end, so that the first buffer member can be easilyprovided.

Fourth Embodiment

In the fourth embodiment shown in FIGS. 8 and 9, the cylindrical member17, as made of rubber to act as the first buffer member, is comprised ofthe cylindrical part 17 a, the angled part 17 b and the annular part 17c, and is formed into a cylindrical tube shape. The cylindrical part 17a covers the outer circumference of the central core assembly 13, theannular angled part 17 b covers the end corner of the permanent magnet15, and the annular part 17 c covers a portion of the longitudinalbottom end face of the permanent magnet 15. The cylindrical part 17 aextends to the circumferential side of the permanent magnet 14, but itsend portion falls short of the top end face of the permanent magnet 14.

Plate members 17 f and 17 g made of rubber to act as the second buffermember are formed into a circular shape separate from the cylindricalmember 17. The plate members 17 f and 17 g are made radially smallerthan the permanent magnets 14 and 15 and are in abutment against thelongitudinal end faces of the permanent magnets 14 and 15, respectively.

As shown in FIG. 8, the end corner of the permanent magnet 14 issurrounded by a space 100 and is kept out of contact with any member.Moreover, the plate member 17 f effects a sealing between the cap 19 asthe case member and the permanent magnet 14 so that the epoxy resin 26will not enter the central core assembly 13.

In the fourth embodiment, the end corner of the permanent magnet 14confronts the space 100, and the end corner of the permanent magnet 15is covered with the cylindrical member 17, so that the two longitudinalend corners of the central core assembly 13 are out of contact with thesecondary spool 20 and the epoxy resin 26. Since the outer circumferenceof the central core assembly 13 is covered with the cylindrical part 17a, moreover, even if the central core assembly 13 and the secondaryspool 20 or the epoxy resin 26 having different thermal expansioncoefficients repeat expansions and contractions in accordance with thetemperature change, the cracks are prevented around the outercircumference of the central core assembly 13 and especially at thesecondary spool 20 and the epoxy resin 26 in the vicinity of the two endcorners of the central core assembly 13, where the cracks mightotherwise be liable to occur, so that the discharge between the highvoltage side and the central core assembly 13 can be prevented. Thismakes it possible to apply the desired high voltage to the ignitionplug.

As a result of the elastic deformations of the plate members 17 f and 17g, moreover, the forces for the central core assembly 13 to receive inthe radial direction and in the longitudinal direction are buffered sothat the magneto-striction will not occur in the central core assembly13. Thus, the desired high voltage can be applied to the ignition plug.Moreover, the plate member 17 f as the second buffer member acts as theseal member between the end face of the permanent magnet 14 and the cap19 so that the number of parts and the number of assembling steps arereduced.

Only the end corner at the side of the permanent magnet 14 is disposedin the space 100 and kept out of contact with other members. However,only the end corner of the permanent magnet 15 may be surrounded by aspace or both of the end corners of the permanent magnets 14 and 15 maybe surrounded by respective spaces.

In the foregoing first to fourth embodiments, at least one of the outercircumference and the two longitudinal end corners of the central coreassembly 13 is covered with the buffer member such as the cylindricalmember 17, and the other is either covered with the cylindrical member17 or made to be surrounded by the space. As a result, the secondaryspool 20 and the epoxy resin 26 having the thermal expansion coefficientdifferent from that of the central core assembly 13 are prevented fromcontacting with the outer circumference and the two end corners of thecentral core assembly 13, and the difference in the thermal expansioncoefficients is absorbed by the elastic deformation of the buffermember. As a result, even if the central core and the secondary spool 20or the epoxy resin 26 having different expansion coefficients repeatexpansions and contractions in accordance with the temperature change,the cracks are prevented around the outer circumference of the centralcore and especially at the secondary spool 20 and the epoxy resin 26 inthe vicinity of the two longitudinal end corners of the central core,where the cracks might otherwise be liable to occur. Thus, the dischargebetween the high voltage side in the ignition coil and the central coreor the low voltage side can be prevented, as might otherwise occur alongthe cracks, so that the desired high voltage can be applied to theignition plug.

Moreover, the outer circumference of the central core assembly 13 iscovered with the cylindrical member 17, and the two longitudinal endfaces of the central core assembly 13 are covered with either thecylindrical member 17 or the plate members 17 e, 17 f, 17 g acting asthe buffer member. Even if the secondary spool 20 or the epoxy resin 26having the thermal expansion coefficient different from that of thecentral core are expanded or contracted together with the central coreassembly 13 as the temperature changes, the cylindrical member 17 andthe plate members 17 e, 17 f, 17 g are elastically deformed to bufferthe forces to be received by the central core assembly 13 in the radialdirection and in the longitudinal direction are buffered. As a result,no magneto-striction will be caused in the central core assembly 13 sothat the desired high voltage can be applied to the ignition plug.

Although the cylindrical member 17 acting as the buffer member isextended in the longitudinal direction of the central core assembly 13and shaped to cover at least one end corner and the outer circumferenceof the central core assembly 13, the buffer member may be comprised of aplurality of members to cover only the longitudinal end corners of thecentral core assembly 13.

Although the cylindrical member 17 and the plate members 17 e, 17 f, 17g are molded of rubber, the cylindrical member 17 and the plate members17 e, 17 f, 17 g can be molded of an elastomer resin, and thecylindrical member 17 can be insert-molded to have the central coreassembly 13 integrally therein. Alternatively, the central core assembly13 may be inserted into the cylindrical member 17 which is molded of theelastomer resin.

Further, the cylindrical member 17 as the buffer member may be providedby covering the surface of the central core assembly 13 with an elasticmember of an elastomer resin or rubber by the integral molding methodsuch as the injection molding, baking or dipping method. In this case,the cylindrical member may cover the whole surface of the central coreassembly 13 or may have a small through hole formed at one longitudinalend portion for discriminating the end specified one end portion of thecentral core assembly 13. By molding the central core assembly 13 andthe cylindrical member 17 integrally, the cylindrical member does notcome out of the central core assembly 13 during the assembling process.

Alternatively, the cylindrical member 17 may be provided by mounting thepermanent magnets 14 and 15 in advance on the core 12 to construct thecentral core assembly 13 and by covering the central core assembly 13with a thermally shrinking tube to shrink this tube thermally.

Further, the cylindrical member 17 contacting with the end corners ofthe central core assembly 13 may be prevented from any damage bychamfering the end corners of the central core assembly 13, i.e., theend corners of the permanent magnets 14 and 15 by polishing or the like.

Fifth Embodiment

In the fifth embodiment shown in FIGS. 11 and 12, at the end portion ofthe primary spool 23, as located at the low voltage side of thesecondary coil 21, there is formed a flange 23 a which is bulgedradially outward and which has a fitting portion 23 b formed to have anL-shaped section for fitting a ring member 50 a therein.

The inner circumference corners of the two longitudinal end portions ofthe outer core 25 are covered with ring members 50 b and 50 a which aremade of rubber to act as angled members. The inner circumference of theend portion of the outer core 25, as located at the high voltage side ofthe secondary coil 21, is covered with the ring member 50, whereas theinner circumference corner of the end portion of the outer core 25, aslocated at the low voltage side of the secondary coil 21, is coveredwith the ring member 51. As shown in FIG. 11, the ring member 50 a isfitted in the fitting portion 23 b which is formed in the flange 23 a.Before the ring member 50 a is fitted in the fitting portion 23 b, theinternal diameter of the ring member 50 a is set to be slightly smallerthan the external diameter of the outer circumference of the fittingportion 23 b. As a result, the elastic force of the ring member 50 aacts upon the fitting portion 23 b inward in the radial direction.

The ignition coil 10 is assembled as follows.

(1) The ring member 50 b is fitted in one end portion of the outer core25, and this outer core 25 is inserted from the side of the ring member50 b into the transformer portion lib having the high voltage terminal41 and the spring 42. The ring member 50 b is retained by the retainingportion 13 a of the transformer portion 11 b, as shown in FIG. 12, toregulate the stroke of insertion of the outer core 25.

(2) The coil assembly, as constructed of the central core assembly 13,the permanent magnets 14 and 15, the secondary spool 20, the secondarycoil 21, the primary spool 23 having the ring member 50 a fitted in thefitting portion 23 b, and the primary coil 24, is inserted into theouter core 25. The ring member 50 a is fitted in the fitting portion 23b by the radially inward elastic force so that it is less likely to getout of place from the fitting portion 23 b. The ring member 50 a isretained on the inner circumference corner of the end portion of theouter core 25 so that the stroke of insertion of the coil assembly isregulated.

(3) The cap is fitted on the transformer portion 11 b, and the epoxyresin is poured from the opening 12 a of a cap 31.

In the assembling procedure described above, the coil assembly includingthe outer core 25 may be inserted into the transformer portion 11 b byassembling the outer core 25 with the coil assembly, and then bycovering the inner circumference corner of the end portion of the outercore 25 at the low voltage side in advance with the ring member 51.

Here, the epoxy resin 26 has a larger thermal expansion coefficient thanthat of the outer core 25 made of a silicon steel sheet. If the innercircumference corners of the two end portions of the outer core 25 arenot covered with the ring members 50 b and 50 a but are in directcontact with the epoxy resin 26, the ring members 50 b and 50 a and theepoxy resin 26 repeat the expansions and contractions as the temperaturechanges, so that cracks will occur in the epoxy resin 26 contacting withthe inner circumference corners of the two end portions of the outercore 25. If the cracks occur in the epoxy resin 26 contacting with theinner circumference corners of the two end portions of the outer core25, a discharge may occur through the cracks between the dummy coil 22,the terminal plate 40 or the high voltage terminal 41 at the highvoltage side of the secondary coil 21 or the high voltage side and theouter core 25 or the low voltage portion. With this discharge betweenthe high voltage portion and the low voltage portion, the voltage to beapplied to the ignition plug drops so that the desired high voltagecannot be applied to the ignition plug.

In the Fifth embodiment, however, the inner circumference corners of thetwo end portions of the outer core 25 are covered with the ring members50 b and 50 a made of rubber, so that they are prevented from contactingdirectly with the epoxy resin 26. Moreover, the difference in theexpansion coefficient between the outer core 25 and the epoxy resin 26can be absorbed by the elastic deformations of the ring members 50 b and51. As a result, no crack occurs in the epoxy resin 26 in the vicinityof the inner circumference corners of the two end portions of the outercore 25 so that the discharge can be suppressed between the high voltageside of the secondary coil 21, i.e., the dummy coil 22, the terminalplate 40 or the high voltage terminal 41 and the outer core 25. As aresult, the desired high voltage can be applied to the ignition plug.

Moreover, the ring member 50 a can be fitted in the fitting portion 23 bof the primary spool 23 so that the ring member 50 a is less likely tocome out of the primary spool 23 when this primary spool 23 is insertedinto the outer core 25. As a result, the assemlability of the ringmember 50 a is improved to reduce the number of assembling steps.

Sixth Embodiment

In the sixth embodiment, at the end portion of a primary spool 27, aslocated at the low voltage side of the secondary coil 21, there isformed the flange 23 a, in which an annular groove 27 b is formed as thefitting portion for fitting the ring member 50 c as the angled member.When the ring member 50 c is fitted in the annular groove 27 b, itslongitudinal motion is regulated so that the ring member 50 c is lesslikely to get out of position when the primary spool 27 is inserted intothe outer core 25. As a result, the assembly of the primary spool 27having the ring member 50 c fitted therein is further facilitated toreduce the number of assembling steps. The inner circumference corner,as located at the high voltage side of the secondary coil 21, of the endportions of the outer core 25 is covered with the ring member 50 b as inthe fifth embodiment.

In the Fifth embodiment and the second embodiment described above, thering member as the angled member covers the inner circumference cornersof the two longitudinal end portions of the outer core 25 thereby toprevent the epoxy resin 26 from coming into direct contact with theinner circumference corners of the two end portions of the outer core25. As a result, the cracks are suppressed in the epoxy resin 26 in thevicinity of the inner circumference corners of the two end portions ofthe outer core 25 due to the temperature change. By making the ringmembers of an elastic material such as rubber, moreover, the differencein the expansion coefficient between the outer core 25 and the epoxyresin 26 is absorbed by the elastic deformation of the ring members sothat the cracks are made further less likely to occur. As a result, thedischarge between the high voltage side of the secondary coil 21 or thehigh voltage portion such as the dummy coil 22, the terminal plate 40 orthe high voltage terminal 41 and the outer core 25 or the low voltageportion can be suppressed to apply the desired high voltage to theignition coil. On the other hand, not the whole surface of the outercore 25 but only the inner circumference corner of its end portion iscovered with the ring member so that the radius of the ignition coil isnot enlarged.

The ring member as the angled member is made of rubber in the fifthembodiment and sixth embodiment, but the rubber may be replaced by anelastomer resin. Moreover, the ring member may be made of a hard resinor the like in place of the elastic material if the inner circumferencecorner of the end portion of the outer core can be covered with a curedface.

If the angled member is made of a volumetrically shrinkable materialsuch as independently foamed sponge, on the other hand, this sponge iseasily deformable so that the sponge abutting against the outer core canbe deformed in its section into an L-shape conforming the shape of theinner circumference corner of the end portion of the outer core byapplying the outer core to the independently foamed sponge thereby tocover the inner circumference corner of the end portion of the outercore. As a result, the angled member can be formed in its sectionalshape not into the L-shape in advance but into the simple plate shape sothat it can be easily worked.

The ring members cover the inner circumference corners of the two endportions of the outer core 25 in the embodiments but can cover only theinner circumference corner of one end portion of the outer core 25.Moreover, with no radial restriction, the end portion of the outer core,as located at the low voltage side of the secondary coil, for example,may be covered with a ring member having a C-shaped section.

Seventh Embodiment

In the seventh embodiment, the inner circumference corner of the endportion of the outer core 25 is not covered with the ring member, butthe end portion of the primary spool 23, as located at the low voltageside of the secondary coil 21, is extended longer in the longitudinaldirection than the outer core 25. Moreover, the flange 23 a, as formedat the end portion of the primary spool 23 at the low voltage side ofthe secondary coil 21, is more extended in the radial direction than theend portion of the outer core 25 thereby to cover the end portion of theouter core 25. The inner circumference corner of the end portion of theouter core 25, as located at the high voltage side of the secondary coil21, is covered with the ring member 50 b (not shown) as in the fifthembodiment.

In the seventh embodiment, the cracks, if caused in the epoxy resin 26in the vicinity of the corner of the end portion of the outer core 25,are shielded by the flange 23 a so that they become less likely toextend. As a result, the cracks fail to reach the electric wiresconnecting the secondary coil 21 and the primary coil 24, and theterminals which are arranged in the ignition coil, so that the electricwires can be prevented from being broken by the cracks. Moreover, thedischarge is suppressed through the cracks between the high voltage sideof the secondary coil or the high voltage terminal and the outer core 25so that the desired high voltage can be applied to the ignition plug.

If the primary spool is extended at its flange as short as the radiallyinner side of the outer core 25 but at its end portion at the lowvoltage side of the secondary coil longer in the longitudinal directionthan the outer core 25, it can prevent the cracks from extending to theinner circumferential side of the primary spool. As a result, thebreakage of the electric wires can be prevented to suppress thedischarge.

In a modification of the shown in FIG. 15, the end portion of the outercore 25 is held in contact with and covered with the flange 23 a of theprimary spool 23. Since the inner circumference corner of the endportion of the outer core 25 hardly contacts with the epoxy resin 26,the cracks are prevented from occurring in the epoxy resin 26, and thecracks, if caused in the epoxy resin 26 in the vicinity of the innercircumference corner of the end portion of the outer core 25, can beprevented from extending.

In the seventh embodiment and its modification, the inner circumferencecorner of the end portion of the outer core 25, as covered with theprimary spool, is not covered with the ring member. However, the endportion of the outer core 25, as covered with the ring member, isfurther covered with the ring member, which is covered with the flangeof the primary spool.

On the other hand, the inner circumference of the end portion of theouter core 25 at the high voltage side of the secondary coil is notcovered with the ring member 50 b but may be covered with the flange ofthe primary spool or the outer spool. When the secondary coil 21 isarranged around the outer circumference of the primary coil 24, too, theinner circumference corners of the end portions of the outer core 25 atthe low voltage side and the high voltage side of the secondary coil arenot covered with the ring members but may be covered with the flange ofthe secondary spool. If the inner circumference corner of the endportion of the outer core 25 at the high voltage side of the secondarycoil is not covered with the ring member, the cracks may occur in theepoxy resin 26 in the vicinity of the inner circumference corner of theend portion of the outer core 25 thereby to establish the dischargebetween the high voltage side of the secondary coil 21 and the outercore 25. However, the cracks, if any, are shielded by the flange of thesecondary spool or the outer spool and are suppressed from any extensionso that the discharge can be suppressed between another high voltageportion and the outer core 25. Moreover, the electric wires, if any atthe high voltage side of the secondary coil, can be prevented frombreaking.

In the above plural embodiments of the invention thus far described, thering member to come into contact with the corner of the end portion ofthe outer core 25 can be prevented from any damage by rounding the sameend portion corner by chamfering it by the indenting or machiningmethod. When the end portion of the corner of the outer core 25 is notcovered with the ring member, too, the cracks can be suppressed in theepoxy resin 26 in the vicinity of the end portion corner of the outercore 25.

The primary coil 24 is arranged around the outer circumference of thesecondary coil 21 in the foregoing plural embodiments, but the secondarycoil 21 may be arranged around the outer circumference of the primarycoil 24.

Eighth embodiment

In the eighth embodiment shown in FIGS. 16 and 17, the primary spool 23is disposed on the outer periphery of the secondary coil 21 and isformed of a resin material. A thin film 51 as a separating member madeof PET (polyethylene terephthalate) for example is wrapped around theouter periphery of the primary spool 23 shown in FIG. 18. The primarycoil 24 is wound around the outer periphery of the thin film 51. Thethin film 51 may be wrapped by overlapping a wrap end 51 a as shown inFIG. 19 or by leaving a gap 51 b as shown in FIG. 20. The thin film 51formed of PET adheres less with both of the primary spool 23 and epoxyresin 26. Accordingly, the primary spool 23 and the primary coil 24 canexpand/contract separately without restraining each other when theprimary spool 23 and the primary coil 24 whose thermal expansioncoefficients differ expand/contract as the surrounding temperaturechanges.

The outer core 25 is attached around the outer periphery of the primarycoil 24. Because the outer core 25 is formed by wrapping a thin siliconsteel plate cylindrically around the primary coil 24 so that its wrapstarting end is not connected with its wrap ending end, a gap isprovided in the longitudinal direction. The outer core 25 extends fromthe peripheral position of the permanent magnet 14 (FIG. 1) to theperipheral position of the permanent magnet 15 in the longitudinaldirection.

In the above eighth embodiment, the thin film 51 interposed between theprimary spool 23 and the primary coil 24 adheres less with the epoxyresin 26 which has infiltrated between coil wires of the primary coil 24and the primary spool 23. Accordingly, when each member of the ignitioncoil 10 expands/contracts as the ambient temperature changes, (1) themembers on the inner periphery side of the thin film 51, i.e., theprimary spool 23, the secondary coil 21, the secondary spool 20, thecentral core assembly 13 and the epoxy resin 26 on the inner peripheryside of the thin film 51 and (2) the members on the outer periphery sideof the thin film 51, i.e., the primary coil 24, the outer core 25, thehousing 11 and the epoxy resin 26 on the outer periphery side of thethin film 51 expand/contract separately from each other bordering on thethin film 51. Thereby, the force which acts on each other when the innerand the outer peripheral parts of the thin film 51 expand/contract isdivided by the thin film 51. Accordingly, the force which acts on theinner peripheral part which is otherwise liable to receive the greaterforce than the outer peripheral part when they expand/contract isreduced, so that the distortion of the inner peripheral part is reduced.For instance, because the distortion of the secondary spool 20 as amember composing the inner peripheral part is reduced, it is possible toprevent the secondary spool 20 from cracking in low temperature when thetoughness of the secondary spool 20 drops. Thereby, it is possible toprevent the electric discharge from occurring between the coil wirescomposing the secondary coil 21 along the crack which might otherwise becaused in the secondary spool 20 and to prevent the electric dischargebetween the secondary coil 21 and the central core assembly 13 as wellas the dielectric breakdown between the secondary coil 21 and thecentral core assembly 13 from occurring. Accordingly, desired highvoltage is generated by the secondary coil 21 and the high voltagecauses the ignition plug to generate a good spark.

Because it is possible to reduce the distortion of not only thesecondary spool 20 but also of the epoxy resin 26 as the innerperipheral part filled between the secondary spool 20 and the core 12caused by the expansion/contraction and to prevent the crack fromoccurring at the surface of contact with the core 12, it is possible toprevent the insulation between the secondary coil 21 and the core 12from being broken.

Ninth Embodiment

In the ninth embodiment shown in FIGS. 21 and 22, the thin film 51 isinterposed between the primary coil 24 and the outer core 25. Althoughthe position of the thin film 51 is different from that in the eighthembodiment, the force which acts on each other when the inner and outerperipheral parts expand/contract bordering on the thin film 51 isdivided by the thin film 51 in the same manner as in the eighthembodiment. Accordingly, it is possible to prevent the member, e.g., thesecondary spool 20, composing the inner peripheral part from crackingand to prevent dielectric breakdown within the ignition coil 10.

Although the PET thin film 51 is used as the separating member in theeighth and ninth embodiments, it is possible to form a separating memberby applying PET as a separating material on the primary spool 23.Instead of PET, silicone, wax or the like may be used as the separatingmaterial to be applied on the primary spool 23. Also a rubber member maybe wrapped around the primary spool 23 or the like or a rubber memberformed in a shape of tube in advance may be fitted on the primary spool23 or the like. Further, a plurality of thin films may be disposed at aplurality of sections.

Although the thin film 51 which adheres less with the spool and theepoxy resin 26 has been used as the separating member in the aboveembodiments, the use of a separating member which adheres less with atleast either one of the spool and the epoxy resin 26 also allows theinner and outer peripheral parts of the ignition coil 10 to be separatedso that those can expand/contract separately from each other borderingon the separating member.

Although the inner and outer peripheral parts of the ignition coil havebeen separated by using the thin film 51 in the above embodiments, thespool itself may be used as a separating member by forming the spool byPPS (polyphenylene sulfide) or PET forming the thin film 51. Thereby,because no separating member needs to be provided anew, the number ofparts and the number of manufacturing steps may be reduced.

Further, it is possible to apply PET, silicone, wax or the like as aseparating material to the primary coil 24 so that the epoxy resin 26will not contact with the primary spool 23. It becomes possible toprevent the resin insulator in contact with the primary coil 24 fromcracking by applying the separating material on the primary coil 24.

Instead of applying the separating material on the primary coil 24, thecoil wires of the primary coil 24 may be coated by a material, e.g.,nylon or fluorine, which does not adhere with the epoxy resin 26.Thereby, the primary coil 24 and the resin insulator 26 canexpand/contract separately, so that the restraint added to the primaryspool 23 via the resin insulator 26 from the the primary coil 24 islowered when they expand/contract. Accordingly, it is possible toprevent the primary spool 23 and the resin insulator 26 in contact withthe primary spool 23 from cracking.

Tenth Embodiment

In the tenth embodiment shown in FIG. 23, the housing 11 of the ignitioncoil 10 has a first housing (transformer portion) 11 a and a secondhousing (plug portion) 11 c, and the connector 30 formed by inserting aplurality of terminals 30 a is provided at an opening on the low voltageside of the first housing 11 b. An electronic igniter circuit 66 as theswitching circuit is provided within the ignition coil 10.

The primary coil 24 is made of a coil wire 71 which is constructed asshown in FIG. 24 before it is wound. The wire 71 is a self-fusing type.An insulating layer 73 is formed on the outer periphery of a copper wirematerial 72 which forms the main body of the wire 71, a separating layer74 of nylon or fluorite is formed on the outer periphery of theinsulating layer 73 as a separating material and a fusing layer 75 of afusing material is formed on the outer periphery of the separating layer74.

The fusing layer 75 melts and the wire 71 adhere each other by heatingafter winding the wire 71 around a temporary core member in a coil. Whenit is cooled in that state, the melted fusing material is solidified andthe wire 71 is combined each other longitudinally, maintaining the shapeof the tubular coil even if it is removed from the temporary coremember. Accordingly, the primary coil 24 may be assembled without usinga primary spool for the primary coil 24.

The primary coil 24 thus formed may be considered to have the samestructure with a coil which is coated by the fusing material by itsouter and inner peripheral sides and which is applied by the separatingmaterial within the fusing material. When the primary coil 24 and theepoxy resin 26 on the inner and outer peripheral sides of the primarycoil 24 whose thermal expansion coefficient differ repeatedlyexpand/contract with changes in temperature, the fusing materialexpands/contracts together with the epoxy resin 26 because the fusingmaterial adheres strongly with the epoxy resin 26. The separatingmaterial adheres less with the fusing material, so that the primary coil24 is separated from the epoxy resin 26 on the inner and outerperipheral sides of the primary coil 24 bordering on the separatingmaterial and can expand/contract separately from each other.

Because the shape of the primary coil 24 can be maintained withoutwinding it around the spool, the primary spool may be omitted and thediameter of the ignition coil 10 may be reduced in the radial thickness.Further, because the primary spool can be omitted, the number of partsand the production cost may be reduced.

Although the separating layer 74 is formed on the inner peripheral sideand the fusing layer 75 has is formed on the outer peripheral side, theseparating layer 74 may be formed on the outer peripheral side and thefusing layer 75 may be formed on the inner peripheral side. Further, onecoating layer which possesses both separating and fusing qualities maybe formed by mixing the separating material and the fusing material. Itis also possible to form one coating layer which possesses bothqualities by one material by using a separating material having thefusing quality or a fusing material having the separating quality. Theseparating member may be disposed on the inner or the outer peripheralside of the coils combined by the fusing material without forming theseparating layer on the wire.

Although the fusing layer 75 is formed only on the primary coil 24 andthe primary spool is omitted, the fusing layer may be formed only on thesecondary coil or may be formed on both primary and secondary coils 24and 21. In this case, the separating layer is formed on the coil onwhich the fusing layer is formed.

Although the secondary coil 21 is provided on the inner peripheral sideof the primary coil 24 in the above embodiments, it is also possible toreverse the position of the primary coil 24 and the secondary coil 21 bydisposing the secondary coil 21 on the outer peripheral side and theprimary coil 24 on the inner peripheral side.

Eleventh Embodiment

In the eleventh embodiment shown in FIGS. 25 and 26, the secondary spool20 is disposed on the outer periphery of the cylindrical rubber member17 and is formed of a resin material. The secondary coil 21 is disposedaround the outer periphery of the secondary spool 20 and is electricallyconnected with the high voltage terminal 41. The primary spool 23 isdisposed around the outer periphery of the secondary coil 21 and isformed of a resin material. The primary coil 24 is wound around theouter periphery of the primary spool 23.

Each of the primary and secondary spools 23 and 20 is molded of theresin material containing at least one of PPE, PS and PBT and whosesolution viscosity is kept to be less than 0.5 and to which more than 5weight % of SEBS (styrene-ethylene-butene-styrene) rubber for example asa rubber component whose glass transition point temperature Tg is −30°or less and glass fibers as a reinforcing material for preventing theplastic deformation of the spool are contained.

As shown in FIGS. 27 and 28, a spool molding die 100 comprises a mainbody 101, an inlet port 102, an outlet port 103 and an alignment plate105. In FIGS. 27 and 28, arrows indicate the direction of flow of theresin.

The inlet port 102, the outlet port 103 and the alignment plate 105forming the path of the resin are formed extending in the axialdirection of the main body 101 which is the molding die of the spoolitself, so that the orientation of the glass fibers within the resin isuniformed across the axial length of the main body 101. A width of thepath of the resin formed within the alignment plate 105 is narrow, sothat the orientation of the glass fibers is liable to go along thedirection of the flow of the resin.

When the resin is injected from the inlet port 102, the glass fiberswhich are oriented almost uniformly along the direction of flow of theresin within the alignment plate 105 are oriented uniformly along theflow of the resin within the main body 101, i.e., along thecircumferential direction thereof, and flows out of the outlet port 103via the alignment plate 105.

Because each spool is molded of the resin material containing at leastone of PPE, PS and PBT and more than 5 weight % of the rubber componentwhose glass transition point temperature Tg is −30° or less to enhancethe toughness of the spool in low temperature, the spool repeatsexpansion/contraction without cracking while adhering with the coil bythe epoxy resin 26 infiltrating between wire rods composing each coileven if the ambient temperature changes. In particular, because thetoughness of each spool may be maintained in low temperature, it ispossible to prevent each spool from cracking in low temperature duringwhich the tenacity is inclined to drop. Accordingly, it is possible toprevent electric discharge from occurring along a crack of the spoolbetween the coil wires composing the coil. Further, it is possible toprevent electric discharge from occurring between the secondary coil 21which is located in the vicinity of the core 12 and generates highvoltage and the core 12 and to prevent dielectric breakdown fromoccurring between the secondary coil 21 and the core 12.

Further, because a fluidity of the resin material drops and it becomesdifficult to mold the spool when the rubber component is added toenhance the toughness of the spool, the drop of the fluidity issuppressed by setting the solution viscosity of the resin material at0.5 or less.

Still more, a thermal expansion coefficient of the spool in the radialdirection is lowered and is made closer to that of the coil by aligningthe orientation of the glass fibers contained in the resin materialmolding the spool along the circumferential direction. Because it allowsthe difference of the thermal expansion coefficient of the spool withthat of the coil to be reduced and the spool to expand/contractconforming to the coil, the distortion of the spool during theexpansion/contraction is reduced and the spool is prevented fromcracking. Further, the disturbance of the orientation of the glassfibers may be suppressed at the confluent section of the injected resinby providing the outlet port 103 in the spool molding die, so that theorientation of the glass fibers may be uniformed along thecircumferential direction of the spool.

FIG. 29 is a characteristic chart showing an effect of the presentembodiment. In FIG. 29, the horizontal axis represents average values αθ(ppm) of the thermal expansion coefficient of the secondary spool 20 inthe circumferential direction at −40° C. to 130° C. in a testing methodconforming to ASTM.D696 and the vertical axis represents extensions ofrupture εf (%) at −40° C.

In FIG. 29, point A represents a product using a material in which 20weight % of glass fibers GF is added to PPE and PS as the spoolmaterial. This results from a molding attained by flowing the materialof the spool in the axial direction. It can be seen from thischaracteristic chart that the spool of this product cracks because itcontains no rubber component, the extension of rupture εf is small andthe thermal expansion coefficient αθ is large. It is noted that theboundary line which decides whether the spool cracks or not is what wasfound by experiments and is expressed as εf=27800αθ−0.349.

Point B shows characteristics of one in which 5 weight % of rubbercomponent is added to the above product. It can be seen that theextension of rupture εf increases and the spool is prevented fromcracking by adding the rubber component to the prior art spool material.Point C also shows characteristics of the spool. That is, although thesame spool material with that of the prior art product is used, thespool has been molded by the above-mentioned method shown in FIGS. 27and 28. Because the glass fibers are oriented along the circumferentialdirection by molding the spool by the method shown in FIGS. 27 and 28,the thermal expansion coefficient αθ in the circumferential direction issmall (α=30 ppm in the present embodiment), thus preventing the spoolfrom cracking.

Point D shows characteristics of the present embodiment. That is, thethermal expansion coefficient αθ in the circumferential direction isreduced and the extension of rupture εf is increased by adding 5 weight% of rubber component to the above product denoted by A and by orientingthe glass fibers in the circumferential direction by the method shown inFIGS. 27 and 28. It can been seen from this point that it is possible tosuppress the spool from cracking by taking either one method of adding 5weight % of rubber component or of orienting the glass fibers in thecircumferential direction.

Although the glass fibers were contained in the resin material in orderto prevent the plastic deformation of each spool in the embodiment, itis possible to contain glass beads or mica, instead of the glass fiber.

Twelfth Embodiment

In the twelfth embodiment shown in FIGS. 30 and 31, the epoxy resin 26is filled around the core 12 and no cylindrical rubber member is used.The molding material and the molding method of each spool are the samewith the eleventh embodiment.

It allows the spool to be restricted from cracking with a change intemperatures in the same manner with the eleventh embodiment and thenumber of parts as well as the number of production steps to be reduced.

Thirteenth Embodiment

In the thirteenth embodiment shown in FIGS. 32 and 33, the epoxy resin26 is filled between the core 12 and the secondary spool 20 and a wire12 a is wound around the outer periphery of the core 12 across the axialdirection. Thereby, the thermal expansion coefficient of the epoxy resin26 which is greater than that of the core 12 is reduced apparently onlyaround the outer periphery of the core 12. Accordingly, the distortionof the epoxy resin 26 caused at the face of contact with the core 12with a change in temperatures is reduced and the epoxy resin 26 may beprevented from cracking.

Further, because a corner section at a stepped portion of the outerperiphery of the core 12 having a laminated structure is covered by thewire 12 a, it is possible to prevent the epoxy resin 26 filled betweenthe core 12 and the secondary spool 20 on the side of core 12 fromcracking.

Although the wire 12 a has been wound around the outer periphery of thecore 12, it is possible to wind a wire formed of a glass fiber aroundthe core 12 or to cover the core 12 by a tube knitted by glass fibers.Further, it is possible to add an additive which reduces the thermalexpansion coefficient of the epoxy resin 26 filled between the core 12and the secondary spool 20 at least in the vicinity of and across allaround the core 12.

Still more, although the epoxy resin 26 which is filled within thehousing 11 as the resin insulator is also filled between the core 12 andthe secondary spool 20, the epoxy resin 26 which is to be solidified asthe resin insulator may be filled only between the core 12 and thesecondary spool 20 and a fluid such as insulating oil may be used forthe insulation between other members.

Although the rubber component has been included in the resin material ofboth the secondary spool 20 and the primary spool 23, the primary spool20 on the outer periphery side may be molded without including therubber component. Further, it is possible to reverse the position of thesecondary spool 20 and the primary spool 23 and to dispose the secondaryspool 20 on the outer periphery side and the primary spool 23 on theinner periphery side. Both of the secondary spool 20 and the primaryspool 23 may be molded by including the rubber component within theresin material and the secondary spool on the outer periphery side maybe molded without including the rubber component.

Still more, although the spool can be suppressed from cracking byenhancing the toughness of the spool and by reducing its thermalexpansion coefficient, it is possible to suppress the spool fromcracking by reducing elastic modulus of the spool in the circumferentialdirection. That is, it is possible to prevent the spool from cracking byabsorbing the distortion by softening the spool itself and by making itextendible. For instance, it is possible to prevent the spool fromcracking by adopting a material containing at least either one ofsilicon, flexible epoxy and elastomer having small elastic modulus asthe material for molding the spool and by reducing the elastic modulusin a testing method conforming to ASTM.D790 to 1 MPa to 1000 MPa. Here,the spool becomes too soft and the windability in winding a coil aroundthe spool drops when the elastic modulus is reduced below 1 MPa.Further, the distortion cannot be absorbed fully when it is greater than1000 MPa.

Although the thermal expansion coefficient αθ of the spool in thecircumferential direction was reduced by orienting the glass fibers inthe circumferential direction, it is also possible to reduce the thermalexpansion coefficient αθ in the circumferential direction by adopting amaterial containing at least either one of PPS, PET, liquid crystalpolymer and epoxy as the material for molding the spool. Specifically,the thermal expansion coefficient αθ in the circumferential direction inthe testing method conforming to ASTM.D696 may be reduced to 10 ppm to50 ppm. It allows the same effect with orienting the glass fibers in thecircumferential direction to be obtained. At this time, the thermalexpansion coefficient αθ in the circumferential direction may be reducedmore readily by using the method shown in FIGS. 27 and 28 incombination.

FIG. 34 is a characteristic chart showing the effect of this time. InFIG. 34, the horizontal axis represents average values of the thermalexpansion coefficient in the circumferential direction in −40° C. to130° C. and coefficients of expansion in the testing method conformingto ASTM.D696 and the vertical axis represents thermal distortion. It canbe seen also from this chart that the thermal distortion can be reducedconsiderably as compared with a spool having a thermal expansioncoefficient (72 ppm) by reducing the thermal expansion coefficient to 10ppm to 50 ppm.

Fourteenth Embodiment

In the fourteenth embodiment shown in FIG. 35, as in the foregoingembodiments, clearances between the individual components, i.e., thecentral core 12, secondary spool 20, secondary coil 21, primary spool23, primary coil 24, outer core 25 and the housing 11 , arevacuum-filled with the resin insulator 26 in the ignition coil 10 toensure electric insulations between the members and to fix the membersthereby to restrict disconnections or cracks due to vibrations.

The insulator 26, if made of epoxy resin, has a cold modulus ofelasticity E (measured by a test method corresponding to ASTMD790) ofabout 8,400 MPa and a thermal expansion coefficient α (an average at theroom temperature to 70° C. in a test method corresponding to ASTMD696)of about 40 ppm. As shown in FIG. 36, the secondary spool 20 if made ofepoxy resin has the maximum heat-cold distortion. Thus, the insulator 26if made of resin takes the maximum cold-heat distortion of the secondaryspool 20. Therefore, to restrict the breakage of the individual membersnecessitates a separating member (e.g., film) or a buffer member (e.g.,the cylindrical member of rubber).

According to various experiments conducted on the basis of the relationbetween the characteristics of the insulator 26 and the cold-heatdistortion to occur in the secondary spool 20, it was ascertained thatthe breakage of the individual members in the housing 11 can berestricted by employing a flexible-insulator made of a silicone resin,urethane resin, flexible epoxy resin or the like.

Specifically, it was ascertained that the breakage of the individualmembers in the housing 11 can be restricted by setting the cold modulusof elasticity E of the insulator 26 no more than 5,000 MPa, and that thebreakage of the members around the central core 12 can be restricted bysetting the cold modulus of elasticity E of the insulator 26 no morethan 10 MPa.

It was also ascertained that the cold modulus of elasticity E of theinsulator 26 is preferred to be no less than 0.1 MPa because the fixingforces of the individual members drop, if the cold modulus of elasticityE of the insulator 26 is lower than 0.1 MPa, so that breakage such asdisconnections or cracks may be suppressed.

On the other hand, it was also ascertained that the insulationdeteriorates, as enumerated in the following Table 1, if the coldmodulus of elasticity E of the insulator 26 is reduced. In case theinsulation raises no serious problem, as exemplified by the ignitioncoil having a relatively low voltage generation or the insulator 26capable of retaining a sufficient insulation distance, the cold modulusof elasticity E is preferred to be lower. In another case (in which thesufficient insulation has to be retained by the insulator 26), it ispreferred that the cold modulus of elasticity E be no less than 10 MPa.

TABLE 1 Conventional Soft Hard Insulator Urethane Silicone Epoxy E (MPa)8,400 3,000 2 15,000 α (ppm) 40 150 200 15 VD (KV)*1) 38 30 21 36 Tg (°C.) 110-130 <T0 <T0 110-130

(Insulator: Epoxy Resin, E: Cold Modulus of Elasticity at NormalTemperature, α: Thermal Expansion Coefficient, VD: Dielectric BreakdownVoltage, Tg: Glass Transition Temperature, T0: Room Temperature)

Here in the Table 1, *1) conforms to the test method JIS.C.2105 with 40needle electrodes buried.

It was ascertained that the cold-heat distortion of the secondary spool20 can be reduced contrary to the foregoing experiments by reducing thethermal expansion coefficient α of the insulator 26 so that the breakageof the individual members in the housing 11 can be restricted withoutusing any separation members or the like.

By setting the thermal expansion coefficient α of the insulator 26within a range of 10 to 30 ppm, the breakage of the individual membersin the housing 11 can be suppressed without using any separationmembers. By especially noting that the iron used for the central core 12has a thermal expansion coefficient α of 11 ppm and that the copper usedfor the secondary coil 21 has a thermal expansion coefficient α of 17ppm, it is ascertained that the breakage of the individual members inthe housing 11 is more restricted by setting the thermal expansioncoefficient α of the insulator 26 within a range of 11 to 17 ppm.

By setting the thermal expansion coefficient α of the secondary spool 20within a range of 10 to 50 ppm, on the other hand, the thermal expansioncoefficients α of the central core 12, the secondary spool 20 and thesecondary coil 21 come close to one another to suppress occurrence ofthe cold-heat distortion due to the temperature change thereby toimprove the durability of the ignition coil 10.

Thus, the insulator 26 is preferred to have a cold modulus of elasticityE of no more than 5,000 MPa or to have a thermal expansion coefficient αof no more than 30 ppm, as described above.

By using the insulator 26 having a cold modulus of elasticity E of nomore than 10 MPa, on the other hand, the breakage of the members aroundthe central core 12 can be restricted without mounting the buffer memberon the central core 12 although the insulation of the insulator 26 isslightly lowered. By thus using no buffer member, the costs forpreparing and assembling the buffer means can be eliminated to furthersuppress the cost for the ignition coil 1.

When the thermal expansion coefficient α of the insulator 26 is to bedetermined, its average at a temperature range of the room temperatureto 70° C. was determined in the test method corresponding to ASTMD696.Thus, the average of the thermal expansion coefficient α can be easilydetermined because the thermal expansion coefficient α is determined interms of the average at a temperature range from the room temperature tothe glass transition temperature of 70° C.

That is, since the insulator 26 has a glass transition temperature Tg,as illustrated in FIG. 37, the average of the thermal expansioncoefficient α is hard to determine if the glass transition temperatureTg is present in the temperature to be averaged. This glass transitiontemperature Tg of the insulator 26 is not present in the temperaturerange from the room temperature to 70° C. so that the average of thethermal expansion coefficient α can be easily determined.

Fifteenth Embodiment

In the fifteenth embodiment shown in FIG. 38, the resin insulator isdivided into inner and outer insulators 26 a and 26 b. The innerinsulator 26 a (e.g., a silicone resin, an urethane resin or a flexibleepoxy resin) contacts directly with the central core 12 and has a coldmodulus of elasticity E within a range of 0.1 to 10 MPa. The outerinsulator 26 b (e.g., a silicone resin, a urethane resin, a flexibleepoxy resin, or a hard epoxy resin having no flexibility) providedradially outside of the inner insulator 26 a has a cold modulus ofelasticity E of no less than 10 MPa.

Here, the inner insulator 26 a and the outer insulator 26 b may beprepared either by charging the inside of the housing 11 separately withthose respective materials, or by coating the outer circumference of thecentral core 12, as having the magnets 14 and 15 mounted thereon, inadvance with the inner insulator 26 a and assembling it in the housing11 and subsequently by charging the inside of the housing 11 with theouter insulator 26 b.

By thus setting the cold modulus of elasticity E of the inner insulator26 a no more than 10 MPa and the cold modulus of elasticity E of theouter insulator 26 b more than 10 MPa, the breakage of the membersaround the central core 12 can be suppressed without mounting any buffermember such as the cylindrical member of rubber around the central core12, and the fixing force of its outer circumference can be strengthenedto restrict the breakage such as the disconnections due to thevibration. A separating member can be eliminated by setting the coldmodulus of elasticity E of the outer insulator 26 b no more than 5,000MPa.

The fifteenth embodiments may be modified by setting the thermalexpansion coefficient α of the inner insulator 26 a within a range of 10to 30 ppm and the thermal expansion coefficient α of the outer insulator26 b more than 17 ppm. By setting the thermal expansion coefficient α ofthe inner insulator 26 a within a range of 11 to 17 ppm, on the otherhand, the thermal expansion coefficient α of the inner insulator 26 acan be brought close to that of the iron of the central core 12 or thecopper wire of the coils 21 and 24 thereby to restrict breakages of theinside members of the ignition coil 10 due to the thermal distortionmore reliably.

Although the foregoing embodiments are exemplified by mounting housing11 on the outer circumference of the outer core 25, the housing 12 maynot be used but the outer core 8 may be used to function as the housing.In this modification, the outer core 25 is sealed in its inside bybaking rubber to its slit.

The present invention should not be limited to the disclosed embodimentsand modifications but covers other embodiments and modifications whichmay be implemented by those skilled in the art.

What is claimed is:
 1. An ignition coil for an engine comprising: acylindrical core; a primary coil and a secondary coil wound around anouter periphery of the core; a primary spool around which the primarycoil is wound and a secondary spool around which the secondary coil iswound, said primary spool and said secondary spool being coaxiallydisposed and extending in parallel so that one of said primary spool andsaid secondary spool is disposed radially within the other of saidprimary spool and said secondary spool; and a resin insulator filledaround the core, wherein a resin material for molding at least saidradially inner one of the primary spool and the secondary spool containsmore than 5 weight % of rubber component and a reinforcing material forsuppressing plastic deformation, whereby said rubber component issubstantially uniformly distributed in the material of said radiallyinner spool and a toughness of said radially inner spool is therebyenhanced.
 2. The ignition coil of claim 1, wherein: the secondary coilis disposed radially within the primary coil.
 3. The ignition coil ofclaim 1, wherein: the resin material contains at least one ofpolyphenylene ether (PPE), polystyrene (PS), and polybutyleneterephthalate (PBT).
 4. The ignition coil of claim 1, wherein: therubber component has a glass transition point temperature Tg which isless than −30° C.
 5. The ignition coil of claim 1, wherein: the resinmaterial for molding said radially inner one of the primary spool andthe second spool has a viscosity which is less than 0.5.
 6. The ignitioncoil of claim 1, wherein the ignition coil is disposed in a plug hole ofthe engine.
 7. The ignition coil of claim 1, wherein the reinforcingmaterial comprises glass fibers.
 8. The ignition coil of claim 7,wherein said glass fibers are oriented along a circumferential directionof said radially inner one of the primary spool and the secondary spool,whereby a thermal expansion coefficient of said radially inner one ofthe primary spool and the secondary spool in the circumferentialdirection is reduced.
 9. The ignition coil of claim 1, wherein saidreinforcing material comprises at least one of glass beads, glass fibersand mica.
 10. The ignition coil of claim 1, wherein each said spool isformed from said resin material.
 11. An ignition coil for an enginecomprising: a cylindrical core; a primary coil and a secondary coilwound around an outer periphery of the core; a primary spool aroundwhich the primary coil is wound and a secondary spool around which thesecondary coil is wound, said primary spool and said secondary spoolbeing coaxially disposed and extending in parallel so that one of saidprimary spool and said secondary spool is disposed radially within theother of said primary spool and said secondary spool; and a resininsulator filled around the core, wherein a resin material for moldingat least said radially inner one of the primary spool and the secondaryspool contains more than 5 weight % of rubber component and areinforcing material for suppressing plastic deformation, wherein: theresin insulator is filled between the core and one of the primary spooland the secondary spool; and further comprising a buffer material havinga coefficient of thermal expansion lower than that of the resin materialis disposed at least in the vicinity of and almost all around the outerperiphery of the core.
 12. An ignition coil for an engine comprising: acylindrical core; a primary coil and a secondary coil wound around anouter periphery of the core; a primary spool around which the primarycoil is wound and a secondary spool around which the secondary coil iswound, said primary spool and said secondary spool being coaxiallydisposed and extending in parallel so that one of said primary spool andsaid secondary spool is disposed radially within the other of saidprimary spool and said secondary spool; and a resin insulator filledaround the core, wherein a resin material for molding said radiallyinner one of the primary spool and the secondary spool contains glassfibers for suppressing plastic deformation, said glass fibers beingoriented in a circumferential direction of said radially inner one ofthe primary spool and the secondary spools, whereby a thermal expansioncoefficient of said radially inner one of the primary spool and thesecond spool in the circumferential direction is reduced.
 13. Theignition coil of claim 12, wherein each said spool is formed from saidresin material.